Field of the Invention
The invention relates to a tissue monitoring apparatus, a tissue monitoring method and an ablation lesion monitoring, measuring, and controlling algorithm incorporating diffuse reflectance spectroscopy (DRS) and/or Arrhenius model thermal denaturation kinetics for determining the characteristics of the lesion or the tissue, especially for identifying the transmurality of the ablation lesion. The invention pertains to a device for and method of real time monitoring of lesion formation as ablation is being carried out.
Background Art
Standard ablation catheters in a cardiac space are typically inherently unstable in operation. Methods incorporating the collection of optical spectra are difficult or flawed due to catheter motion and inconsistent contact pressure. Any method incorporating diffuse reflectance spectroscopy (DRS) in a catheter would suffer from these issues. Furthermore, DRS deployed with a standard offset between illuminating and collecting fibers will only penetrate tissue to small penetration depths. Standard catheters cannot achieve a sufficient spatial offset between illuminating and receiving optic fibers due to the limited length of the distal tip of the catheter that is in contact with the heart and therefore cannot exploit a sufficient spatial offset to achieve greater penetration depths.
In ablation of cardiac tissue, the “first hit” (first ablation attempt) is the most important. Therefore applying energy in the correct dosage and for the correct duration the first time is critical to achieving transmurality. Transmurality is critical to a successful outcome.
Insufficient energy and duration will lead to ineffective lesions which result in local tissue swelling and edema which will prevent further effective completion of the lesion at the same location. Checking for lesion characteristics after the first ablation is therefore ineffective. On the other hand, applying too much energy for too long can lead to collateral damage like coagulum formation and damage to surrounding structures.
Including for these reasons, there is a 40% recurrence rate with current ablation technology. One of the primary failure mechanisms is when the lesions are not transmural, that is they do not penetrate the entire way through the tissue and the errant electrical signals can still pass. On the other hand, applying too much energy creates collateral damage to the heart. Thus, the precise application of the right amount of energy is one key to a successful ablation. Unfortunately, it is also very hard to measure, and even worse, the doctor has to get it right the first time.
It has been shown that the recurrence is typically at the deepest tissue layer (Kowalski et al). Thus, it is key that the physician be able to track the lesion formation at deeper tissue levels. When the lesion is formed by the application of RF energy through an electrode, for example, the lesion first forms closest to the electrode. The lesion only later extends to the depths of the tissue. Thus, it is tracking when the lesion reaches the far side of the tissue that is key. This is where the prior art fails. In particular the prior art is unable to provide a catheter that can examine deep within the tissue.
What is therefore needed is a device and method to allow monitoring of the ablation lesion as it is being formed. What is further needed is the ability to adjust power and duration settings according to predetermined optimum levels for achieving transmurality and for preventing collateral damage.
Systems which attempt to measure ablation lesions are known in the art. Certain of these systems rely on optical data. However, quantifying optical sensing data in a beating heart is challenging. There is substantial movement of the catheter. The translation of the catheter can be as much as 1 cm within the timeframe of a single heartbeat. This movement can lead to significant error in spectrophotometer readings and even ultrasound images.
Harks et al., (WO 2012/049621 A1), hereinafter “Harks,” incorporates an optical fiber within a standard ablation catheter. This catheter is subject to cardiac motion—leading to significant errors in spectrophotometer readings. The catheter is also subject to different contact pressures which can lead to significant errors in spectrophotometer readings. Furthermore, in moving catheters around the cardiac atria there is a wide variation in the amount of pressure applied to the tissue.
Kuck et.al; A novel radiofrequency ablation catheter using contact force sensing: Toccata study. Heart Rhythm 2012 Jan. 9(1): 18-23 hereinafter “Kuck,” shows that there is a large inter-individual and intra-individual variability in ablation catheter contact pressure within the left atrium. Even within the various contact levels made by a single individual clinician during a single procedure, there is a wide variability in contact pressure as the catheter is moved from point to point in the heart. This contact pressure can differ by as much as up to 40 g as the ablation catheter is moved around just the pulmonary veins. In addition, depending in the individual clinician performing the ablation, a large variation in contact force of up to 60 g is possible
Irrespective of the source, variability in pressure against tissues can influence measurement of optical sensing data. Variations in pressure on tissue can lead to variations in scattering and absorbance coefficients by as much as up to 25% due to extrusion of water from tissue.
Accordingly, the prior art also lacks a stable platform providing a stable contact pressure for measuring the lesion.
In addition, with past ablation systems such as those used for atrial fibrillation and ventricular tachycardia, there is no method of checking for transmurality. It is up to the operator to decide (or guess) when the lesion is transmural. The result is a significant variation in power delivery and duration of power delivery between different operators. As a result of this, there is a significant recurrence rate of arrhythmias, between 30 to 50 percent for atrial fibrillation. For ventricular tachycardia, the recurrence rate is even higher because ventricular tissue is thicker and so it is harder to achieve transmurality.
Studies have shown that when inpatients come back with recurrence of atrial fibrillation, the pulmonary veins typically have electrically reconnected, proving the inability of current ablation systems to achieve transmurality.
While the atrial tissue may be as thick as 5 mm, histology studies have shown that a rim of unablated tissue as little as 1.4 mm can lead to reconnection of pulmonary veins and lead to recurrence of atrial fibrillation.
In ablation catheters adapted for renal denervation where the sympathetic nerves on the surface of the renal artery are the targets for ablation, there is currently no ablation endpoint. As such, the operator is not able to discern when the nerves have been ablated. As such there have been reports of thrombus formation and even dissection of the renal artery, due perhaps to delivering too much power for too long.
Accordingly, It would be desirable to provide a system that is configured to interrogate deeper layers of tissue to select out the layer of tissue for ablation and to observe for transmurality with NIR spectroscopy and that can provide a constant contact force and also reduce the effect of the beating motion